Benefits of Using Heat Recovery Chillers as They Contribute to Decarbonization and Building Electrification
Introduction to heat recovery chiller systems
As the need for conditioning the indoor environment expanded, the development of the first variable air volume (VAV) system came onto the scene. The first systems utilized electric resistance coils heating to reheat the individual spaces.
Electric strip heating was easy to install and control with relatively low first cost. However, it was extremely inefficient and the utility costs quickly skyrocketed. In response to this, the market shifted to natural gas boilers systems with hot water reheat at the zones. These systems were much more efficient and provided excellent space level control. Now the market is changing again to embrace electrification and decarbonization of buildings without these natural gas boilers. There are several ways to fill this gap left from the boilers and this paper focuses on using heat recovery chiller systems.
Buildings account for nearly 40% of total U.S.energy consumption. Thirty-seven percent of the commercial building energy consumption is attributed to space and water heating.1
What is a heat recovery chiller?
Heat recovery chillers are a type of HVAC system that can simultaneously provide heating and cooling to a building by recovering waste heat from the cooling process and using it to produce hot water for space heating, domestic hot water, or other process requirements, thereby offsetting heating energy.
Heat recovery chillers are commonly used in commercial and institutional buildings such as hospitals, universities and hotels. They are particularly useful in buildings where there is a high demand for both cooling and heating. During cooling only operation, the chiller produces a controlled source of chilled water leaving the evaporator while dissipating heat through the condenser to the environment. When there is a simultaneous need for chilled water and hot water, these chillers have the capability to operate in heat recovery mode.
Typical leaving condenser water temperatures range from 90˚F up to 140˚F:
- Lower leaving condenser water temperature up to 120˚F can be used for VAV reheat, radiant floor heat, pool heating and pre-heating domestic hot water.
- Higher leaving condenser water temperature up to 140˚F can be used for domestic hot water, laundry and other process application.
Leaving condenser water temperature must be higher than heating loop return water temperature to recover heat.
Total coefficient of performance
The total coefficient of performance (COP) for heat reclaim from a chiller system for both water heating and cooling purposes can reach up to 7.0.
What are the benefits?
- Heat recovery captures energy that would have been wasted during the cooling process. The recovered heat can be redirected for various heating needs, which saves energy while maintaining design conditions.
- Unlike the process of generating heat from combustion or electric driven water heaters, capturing waste heat from the condenser can result in high system efficiency.
- Heat recovery chillers may reduce the size of primary heating equipment, or eliminate the primary heating equipment.
- Flexible sizes are available for small or large commercial applications.
- Electrification of heating system provides opportunity for decarbonization.
What are the challenges/constraints?
- Additional components and piping may be needed to accommodate the heat recovery chiller system which may lead to greater first cost investment.
- Higher condenser leaving water temperature will decrease the cooling efficiency and capacity of the chiller, which typically results in a larger chiller requirement.
- Peak electric demand from the building may be increased as systems look to remove natural gas.
- Additional controls may be required.
There are several types of heat recovery chillers available to meet varying requirements of the hot water systems, (service, domestic, or process) or to meet the client’s energy and decarbonization criteria.
1. Single condenser heat recovery chiller
These chillers are the most similar to a conventional chiller. They are used where leaving water temperatures of up to 120˚F are needed for non-potable water applications, such as building heating and process water systems. It is common to pair these chillers with a heat exchanger to separate the cooling tower and the chiller. The chiller’s control system works to maintain the desired “hot” recovered water while simultaneously producing a source of chilled water.
2. Double condenser heat recovery chiller
A dual condenser heat recovery chiller, has two separate condensers with one being dedicated to the heat recovery process. They can recover water upwards of 110˚F, or be able to operate in cooling only if required. These chillers have more complex controls to maintain the leaving “hot” water temperature while maximizing efficiency.
3. Heat recovery chiller in parallel
A common way to integrate a heat recovery chiller is to install it in parallel to the cooling only chillers in a traditional primary/secondary loop configuration. This is applicable to both constant and variable flow systems.
The benefit of this design is the ability to control both hot and chilled water temperatures while maximizing both cooling and heat recovery efficiencies. While there are some additional particularities with each constant and variable flow systems, in general the heat recovery chiller is prioritized over the other chillers to maximize the benefits. Chiller plants set up in configuration also provides the same returning evaporator entering temperatures across each equipment.
Primary/secondary chilled water system with heat recovery chiller
Variable primary flow chilled water system with heat recovery chiller
4. Heat recovery chiller in series
When the heat recovery chiller is set up in series, the heat recovery chiller is primarily loaded and reduces the entering evaporator temperature for the rest of the chillers in system. In this scenario, the heat recovery chiller can be controlled to match the necessary heating loads, while the cooling only chillers will operate a more efficient part loads due to the reduced lift. A bypass pipe is often installed around the heat recovery chiller.
Series counter-flow chilled water system with heat recovery chiller
In continued efforts for increased electrification of buildings and decarbonization of the grid, CEDA is offering inducements for the installation of heat recovery chillers. Primarily focused on systems where the rejected heat is used for space heating applications. These systems should aim to recover 20-30% of the peak heating load or 70% of the annual heating load. Both systems with high efficiency gas boilers or heat pump water heaters are eligible to receive the inducement. The inducements are proportional to the estimated therms savings.